These fish tacos with a crunchy slaw and creamy, tangy sauce really hit the spot. Key limes are smaller, tarter and more aromatic than Persian limes and worth seeking out, but you can definitely use regular limes instead.
For the slaw: Combine the cabbage, lime juice, scallions, jalapeno and cilantro in a large bowl. Season to taste with salt and pepper. Let sit for 10 minutes, toss again and season again to taste with salt and pepper. Set aside for up to 2 hours at room temperature, or refrigerate for up to 2 days. (The longer it sits the better it gets.)
Preheat the grill over medium-high heat to 375 degrees F or heat a grill pan over medium heat. Clean and oil the grill grates.
Meanwhile, for the key lime sauce: Whisk the lime juice, mayonnaise and mustard together in a small bowl. Set aside.
For the fish and assembly: Place the fish on a rimmed baking sheet. Drizzle both sides with oil then liberally sprinkle with cumin, salt and pepper until both sides are coated. Grill the fish until you see grill marks and the flesh is white and easily flakes with a fork, 2 to 3 minutes per side depending on the thickness. (It's okay if the fish breaks since it will be broken up into large chunks for the tacos.) Transfer to a plate.
Grill the tortillas until lightly charred, about 1 minute per side.
Fill each tortilla with about 1/4 cup of the slaw, followed by some of the fish broken into large chunks. Drizzle with sauce, top with a scattering of the tomatoes and a sprinkling of cilantro leaves and serve immediately.